“Beam chemistry” referred to chemical reactions initiated by a beam, such as a charged particle beam or a laser beam. “Electron beam chemistry” includes electron beam-induced deposition (EBID) and electron beam-induced etching (EBIE) and is typically performed in a scanning electron microscope (SEM). In both these electron beam processes, molecules of a precursor gas are adsorbed onto a work piece surface. An electron beam is directed at the work piece, and the electrons dissociate the adsorbates, generating reaction products. In EBID, non-volatile reaction products remain on the substrate surface as a deposit, while volatile reaction products desorb. In EBIE, one or more of the precursor molecule decomposition products react with the work piece surface, generating volatile reaction products that desorb from the work piece, removing surface material. Similar processes occur in ion beam-induced deposition (IBID) and ion beam-induced etching (IBIE), although the much greater mass of the ions also causes material to be removed from the substrate by sputtering, that is, by momentum transfer from the energetic ions, without any chemical reaction. The mechanism by which the ion beam interacts with the adsorbate is thought to be different from the mechanism by which the electron beam reacts with the adsorbate.
To be useful as a precursor gas the molecules need to have very specific properties: they need to stick to the surface for a sufficient time to be dissociated, but must not form a thick layer that shields the surface from the beam, and they should not react spontaneously with the work piece surface material in the absence of the beam. In the case of etching, the precursor dissociation products should form a volatile compound with the substrate material and in the case of deposition, the precursor should decompose in the presence of the beam to deposit the desired material. Other reaction products should be volatile so that they do not remain on the surface and can be removed from the system by a vacuum pump.
Beam chemistry driven by a charged particle beam is typically performed in a vacuum chamber using a gas injection system having a capillary needle that directs gas toward the impact point of the beam. The gas expands rapidly and while the local gas pressure at the surface is sufficient to support beam-induced reactions, the pressure in the rest of the sample chamber is sufficiently low that secondary electrons can be detected using a conventional detector, such as the scintillator-photomultiplier combination commonly referred to as an Everhart-Thornley detector.
Electron beam chemistry can also be performed with a work piece in an environment flooded with the precursor gas, with most of the beam path separated from the gaseous environment by a pressure-limiting aperture. Because the gas pressure does not permit conventional secondary electron detection for imaging, imaging can be performed using gas cascade amplification in which secondary electrons from the sample are accelerated and ionize gas molecules. Electrons from the ionized gas molecules are accelerated and ionize other gas molecules, in a cascade that greatly amplifies the original secondary electron signal. A system in which the sample is maintained in a gaseous environment is typically called an environmental scanning electron microscope or a high-pressure scanning electron microscope (HPSEM). Gases that are not readily ionized are not useful for forming an image using gas cascade amplification. Gases that are susceptible to dielectric breakdown are also not useful for forming an image using gas cascade amplification.
XeF2 has, to date, been the most commonly used precursor gas for beam-induced etching. However, XeF2 has some undesirable effects. XeF2 spontaneously etches many materials, including silicon and TaN. XeF2 is not an optimal HPSEM imaging medium in that it provides poor charge stabilization and poor image quality during EBIE processing. XeF2 is highly corrosive and toxic. XeF2 cannot be mixed with many common gases used for residual carbon removal and surface species control. Moreover, large quantities of XeF2 cause instability in some differentially pumped electron beam systems because of poor ion getter pumping of xenon.
An electron beam can also be used in electron beam lithography. The electron beam exposes photoresist as the beam scans in a pattern. Either exposed areas or unexposed areas, depending on the properties of the resist, are then removed, leaving a patterned photoresist layer. Ice can be used as a patterning material, with the electron beam causing the ice to sublimate in exposed areas, as described, for example, in King et al., “Nanometer Patterning with Ice,” NANO Letters, Vol. 5, No. 6, pp. 1157-60. (2005). Areas from which the ice has been removed can be subject to further processing, such as diffusion or metallization, while other surface areas are protected by the ice layer.
In Gardener et al., “Ice-Assisted Electron Beam Lithography of Graphene,” Nanotechnology 23 (2012) 185302, a thin layer of ice condensed on the surface acts as an EBIE precursor for etching a graphene layer underlying the ice. The electron beam is thought to induce a reaction between the carbon in the graphene and the hydrogen and oxygen in the ice to form volatile carbon compounds that leave the surface, removing the graphene.
In EBID, the dissociation product remains on the material, so the process can be carried out at low temperatures, at which the precursor gases adsorb more readily. In EBIE, however, the volatile reaction byproducts must desorb from the surface, and low temperatures reduce the thermal desorption rate of the reaction products and are therefore considered undesirable.
Bozso et al., “Electronic Excitation-Induced Surface Chemistry and Electron-Beam-Assisted Chemical Vapor Deposition,” Mat. Res. Soc. Symp. Proc., Vol. 158, pp. 201-209 (1990) describes a method of depositing silicon, silicon nitride, silicon oxide, and silicon oxinitride films onto a silicon substrate using low energy EBID at a temperature of approximately 100 K (−173.degree. C.). The deposition method of Bozso is used to separate dissociation reactions caused by electrons from those caused by heat for more precise control over spatial growth and material composition.
U.S. Pat. Publication No. 2012/0003394 of Mulders et al. for “Beam-Induced Deposition at Cryogenic Temperatures,” which is assigned to the assignee of the present invention, teaches choosing a precursor gas from a group of compounds having a melting point that is lower than the cryogenic temperature of the substrate and a sticking coefficient that is between 0 and 1 at the desired cryogenic temperature. This is thought to result in the precursor gas reaching equilibrium between precursor molecules adsorbed onto the substrate surface and precursor gas molecules desorbing from the substrate surface at the desired cryogenic temperature before more than a small number of monolayers of the gas are formed. Suitable precursor gases include alkanes, alkenes, or alkynes, or the branch derivatives of those compounds.
It would be useful to find a method for EBIE that is not associated with the problems described above.